The polyps build the reefs by slowly secreting calcium carbonate over thousands of years, creating structures that can dull the blow hurricanes deal to coastal cities and are vital nurseries for fish.

The polyps also give the reefs their dazzling shades of pink and purple that delight scuba divers and boost tourism from the Great Barrier Reef of Australia to the Florida Keys.

The reefs are also a source of tourism, generating billions of dollars worldwide each year. Some scuba instructors are concerned about the future of the industry.

Jorge Olivieri, who has been taking tourists out diving in the area for the last 16 years, says some reefs are so damaged he would not take an experienced diver to see them.

“There are still fish and coral, but it isn’t like it used to be,” Olivieri said.

But the Global Coral Reef Monitoring Network reports that the amount of reef surface covered by live coral in the Caribbean has fallen about 80 percent in the last three decades.

In the Pacific between Hawaii and Indonesia, reefs have been losing about 1 percent of their coral coverage annually over the last 25 years.

It is hard to tell how much of that damage was caused by global warming and how much by local factors like pollution.

Once a hardly inhabited strip of sand off Mexico’s Yucatan peninsula, Cancun has erupted into a favored place of travel for millions of people each year.

In Cancun’s urban sprawl on the mainland, where hotel and bar workers live, infrastructure has failed to keep up with a ballooning population of around half a million.

The lagoon next to the hotel strip is murky and gives off a foul odor in parts. Only crocodiles swim there now.

“It’s kind of gross,” said U.S. college student Leah, 19.

Away from the lagoon, seawater samples from around Cancun show the levels of chemicals from human waste have increased steadily over the last decade, said Jorge Herrera, a marine biologist at the Cinvestav research center in the nearby city of Merida.

But Cancun’s waste treatment plants do not clean sewage enough to make it safe for coral, marine biologists say. The treatment plants kill bacteria that can be harmful to people but do not remove chemicals like phosphates.

The treated sewage is deposited underground but seeps through the porous soil into the lagoon and the ocean, scientists say. “Little by little, this causes the coral to die,” said Herrera.